The present exemplary embodiment relates to tone reproduction curve control in an electrophotographic printing system, and it will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Electrophotographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In xerographic systems, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. In ionographic print engines the latent image is written to an insulating member by a beam of charge carriers, such as, for example, electrons. However it is created, the latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a copy sheet. The copy sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
In xerographic print engines, a tone reproduction curve (TRC) is important in controlling the image quality of the output. An image input to be copied or printed has a specific tone reproduction curve. The image output terminal outputting a desired image has an intrinsic tone reproduction curve. If the image output terminal is allowed to operate uncontrolled, the tone reproduction curve of the image output by image output terminal will distort the rendition of the image. Thus, an image output terminal must be controlled to match its intrinsic tone reproduction curve to the tone reproduction curve of the input image. An intrinsic tone reproduction curve of an image output terminal may vary due to changes in such uncontrollable variables such as humidity or temperature and the age of the xerographic materials, i.e., the numbers of prints made since the developer, the photoreceptor, etc. were new.
Solid developed mass per unit area (DMA) control is a critical part of TRC control. If the DMA is too low then the images will be too light and customers will be dissatisfied. On the other hand, if the DMA is too high, then other xerographic or image quality problems, such as poor transfer efficiency, fusing defects, or toner scatter on lines, etc., can occur. High DMA will also increase the TCO (Total Cost to Owner). Maintaining a constant DMA or a low variation of DMA has always been a challenge in xerographic process controls design. Low cost reflection sensors, such as black toner area coverage (BTAC) sensors, cannot sense solid DMA due to sensor saturation at high masses. Currently, there are several different kinds of strategies to control DMA.
For example, one strategy has been to use Vdev (development voltage) to control DMA. However, it is hard to control DMA within a small range since it may require a different Vdev to achieve a similar DMA in different environmental zones or with different hardware configurations. Additionally, using measured Ve (image voltage) to calculate Vdev means some toner waste.
Another strategy has been to use a transmission densitometer to measure transmission density (Dt) from the photoreceptor belt in real-time. The Dt is used to infer the DMA. However, the transmission densitometer is about eight to ten times more expensive than that of reflection sensors.
It is obvious that an improved method and system for controlling TRC and DMA by using a low cost reflection sensor, such as a black toner area concentration (BTAC) sensor, in products has significant benefits.